In 1993, Iijima and Bethune reported the synthesis of high aspect ratio (e.g.&gt;1,000), small diameter (eg. 1-2 nm) single-walled carbon nanotubes (SWNTs) prepared in 1-2% yield from carbons generated in a catalyzed carbon DC arc discharge process.sup.1. In 1996, bulk synthesis of approximately 100 mg/day of nearly defect-free SWNTs having a narrow diameter distribution centered about a diameter of approximately 1.36 nm was reported by Smalley at Rice University.sup.2. Using pulsed (YAG) laser vaporization of a heated (eg. approximately 1,200.degree. C.) carbon target containing 1-2% Ni/Co catalyst, the group was able to produce yields of SWNTs of approximately 70-90%. The SWNTs were found to grow in well ordered bundles with an intertubal spacing of approximately 0.30 nm. Also in 1997, Bernier and a group at Montpellier, France reported that an arc discharge between a Ni:Y catalyzed carbon anode and a pure carbon cathode produced a cathode deposit or collaret which also contained yields of SWNTs of approximately 70-90%. FNT .sup.1 C. Journet et al., Nature, 388, 756 (1997). FNT .sup.2 A. Thess et al., Science 273, 483 (1996).
The individual SWNTs produced by these methods may be visualized as a single graphene sheet rolled into a hollow, seamless tube. SWNTs currently produced already have been observed in TEM to exhibit a very high (.about.10.sup.3 -10.sup.4) aspect ratio. A typical average diameter is in the range 1-2 nm, close to that of a DNA molecule. The SWNTs are self-assembled into bundles containing tens to hundreds of individual tubes. The energy band structure of the tubes depends critically on details of the arrangement of the carbon hexagons in the wall relative to the tube axis. Depending on the integers (n,m) that mathematically define this structure (or "chirality"), about 1/3 of the tubes are expected to be metallic, and the remaining 2/3 should be semiconducting with a typical energy gap of .about.0.6 -0.7 eV, in the diameter range 1-2 nm.sup.3. FNT .sup.3 M. S. Dresselhaus, G. Dresselhaus, and P. C. Eklund, Science of Fullerenes and Carbon Nanotubes, (Academic Press, New York), 1996.
In addition to being high-aspect ratio metallic or semiconducting nanofibers, SWNTs have also been found both theoretically and experimentally to exhibit extremely high mechanical strength (strength to weight ratio is .about.400 times higher than in steel) and very high flexibility (.about.30% strain to failure).sup.4. Experimentally, metallic C-SWNTs have been observed via their signature in electrical resistivity experiments.sup.5, 6, 7, their STM density of states .sup.8, and also by their thermoelectric power.sup.6, 9. Semiconducting tubes can be converted to metallic tubes by charge transfer doping discovered by the groups of J. E. Fischer (U. Penn.).sup.5 and P. C. Eklund.sup.10. This is a particularly important discovery, as a real ensemble (sample) of SWNTs can then be completely converted by chemical treatment to either a collection of P- or n-type metallic tubes. FNT .sup.4 B. I. Jakobson, C. J. Brabec, and J. Bernholc, Phys. Rev. Lett. 76, 2511 (1996). FNT .sup.5 R. S. Lee, H. J. Kim, J. E. Fischer, A. Thess, and R. E. Smalley, Nature 388, 255 (1977). FNT .sup.6 L. Grigorian, K. A. Williams, S. Fang, G. U. Sumanasekera, A. L. Loper, E. C. Dickey, S. J. Pennycook, and P. C. Eklund, Phys. Rev. Lett 80, 5560 (1998). FNT .sup.7 L. Grigorian, G. U. Sumanasekera, A. L. Loper, S. Fang, J. L. Allen, and P. C. Eklund, Phys. Rev. B 58, R4195 (1998). FNT .sup.8 J. W. G. Wildoer, L. C. Venema, A. G. Rinzler, R. E. Smalley, and C. Dekker, Nature 391, 59 (1998). FNT .sup.9 J. Hone et al., Phys. Rev. Lett. 80, 1042 (1998). FNT .sup.10 A. M. Rao, P. C. Eklund, S. Bandow, A. Thess, R. E. Smalley, Nature 388, 257 (1997).
The main problem hindering the utilization of this unique combination of properties is that 2/3 of all nanotubes are not metallic, resulting in relatively high electrical resistivity for the as-grown SWNT samples. Preliminary estimates for .rho.(300 K) range from .about.600 .mu..OMEGA. cm in SWNT mats to .about.100 .mu..OMEGA. cm in individual SWNT ropes, i.e, about 80 times higher than in elemental metals such as copper.
Charge transfer doping, or intercalation of SWNT ropes with either alkali metals (K, Cs).sup.5,7 or halogens (bromine, iodine) have been found to induce remarkable 30 to 120-fold drop in resistivity of SWNTs, thereby bringing .rho. (300K) values down to a level comparable to that of copper. Unfortunately, most of the intercalated SWNTs (with iodine as a noteworthy exception) are not ambient-stable and, therefore, their application potential is severely limited. Poor air stability is inherent to most intercalated carbon systems, as the intercalant usually can as easily diffuse out, as it had diffused into, the host. In addition, some intercalants, e.g., alkali metals, react with atmospheric moisture and oxygen leading to irreversible degradation of electrical properties of the intercalated carbon sample. A need is therefore identified for an improved method of doping SWNTs, wherein the ambient stability of the doped SWNTs is enhanced. FNT .sup.5 R. S. Lee, H. J. Kim, J. E. Fischer, A. Thess, and R. E. Smalley, Nature 388, 255 (1977). FNT .sup.7 L. Grigorian, G. U. Sumanasekera, A. L. Loper, S. Fang, J. L. Allen, and P. C. Eklund, Phys. Rev. B 58, R4195 (1998).